Angular velocity sensors are installed in, for example, digital cameras, video cameras, mobile phones, car navigation systems, and any other electronic equipment for detecting a physical quantity changing with external factors. Such angular velocity sensors detect a physical quantity such as angular velocity for use in, for example, image stabilization for digital cameras and self-contained navigation for attitude control. They are also used in navigation using GPS (Global Positioning System).
Angular velocity sensors using piezoelectric elements are known as one of angular velocity sensors. Angular velocity sensors using piezoelectric elements are installed in many devices.
An angular velocity sensor in Japanese Utility Model Registration No. 3135181 (Patent Literature 1) and Japanese Patent Laying-Open No. 8-35981 (Patent Literature 2) includes an annular flexible portion, a mass portion supported on an inner edge of the flexible portion, a support portion supporting an outer edge of the flexible portion, and first and second piezoelectric elements. The flexible portion is arranged along an XY plane such that the Z axis passes through the center thereof. The first piezoelectric element is provided on the positive side of the X axis (or Y axis) and fixed on the front surface of the flexible portion. The second piezoelectric element is provided on the negative side of the X axis (or Y axis) and fixed on the front surface of the flexible portion. This angular velocity sensor obtains the angular velocity around the Y axis (or X axis) with the mass portion vibrated in the Z axis direction, based on the quantity of charge generated at each of the first and second piezoelectric elements.
In an angular velocity sensor in Japanese Patent Laying-Open No. 7-167660 (Patent Literature 3), first to third piezoelectric elements are fixed to a gyro oscillator. The first piezoelectric element is used to vibrate the gyro oscillator, the second piezoelectric element is used to detect vibration of the gyro oscillator, and the third piezoelectric element is used to detect Coriolis force. The output signal of the second piezoelectric element is amplified and delayed by 90 degrees to generate a driving signal for the first piezoelectric element.
In the angular velocity sensor using piezoelectric elements, the electric charge output by the piezoelectric element is converted into voltage, and an angular velocity signal component is selectively extracted from a sensor signal containing a driving signal component by a synchronous detection circuit and is then converted into direct current. After the detection output is smoothed by a low-pass filter, the DC voltage signal is amplified by an amplification circuit for output.
In such angular velocity sensors, electric charge output by piezoelectric elements becomes extremely weak as the oscillator is reduced in size and weight. Therefore, it is requested for the devices for detecting the angular velocity based on such weak electric charge to be able to detect the angular velocity with no distortion and low noise and with proper gain.
For example, Japanese Patent Laying-Open No. 2009-168588 (Patent Literature 4) describes a synchronous detection circuit which cancels offset voltage, which is noise, from a sensor signal.
In Patent Literature 4, the synchronous detection circuit synchronously detects a sensor signal in synchronization with a sampling pulse for synchronous detection, which has a pulse width shorter than half the cycle of the sensor signal and has the same cycle as the cycle of the sensor signal. In the synchronous detection circuit, the ratio of a detection output with respect to an angular velocity signal component is reduced by removing part of the angular velocity signal component, thereby cancelling positive offset voltage based on DC bias voltage of a signal path.
Japanese Patent Laying-Open No. 2009-128135 (Patent Literature 5), paragraph 116, describes that the S/N ratio can be improved by performing sampling in the vicinity of a period in which the displacement of an angular velocity signal component is greatest.
Japanese Patent Laying-Open No. 2008-224230 (Patent Literature 6), paragraph 93, suggests a technique of adjusting a pulse width of a sampling pulse.
Japanese Patent Laying-Open No. 10-234094 (Patent Literature 7) does not relate to an angular velocity sensor but relates to a speaker device. Patent Literature 7 discloses that hearing decreases in treble frequencies higher than 1 KHz. In the angular velocity sensor of the present invention, it is useful to know the frequency range in which hearing decreases, in determining the frequency of a driving signal.